The musical instrument concerned is in particular a pipe organ which has one or more manual keyboards (or manuals), a pedal-board, and stops for as many pipes or ranks of pipes as there are keys (or pedals) on the keyboard (or pedal-board) with which they are associated.
These organ stops are mounted on a common wind-chest which allows any pipe to emit a sound if the appropriate key or pedal of the keyboard or pedal-board is pressed and if the stop serving the pipe is selected by means of an individual control element or actuator which is generally arranged opposite the corresponding keyboard.
The instrument also includes various couplers which enable one or more keyboards in a position n+k (k=1, 2, etc.) to be operated when a keyboard in position n, or the pedal-board, is operated. Finally it includes one or more bellows which supply air at a regulated pressure.
The pipes are caused to operate by means of valves which are controlled mechanically or electromechanically from the keys, the pedals and the stops.
In the older type of organ, the stops are selected directly by the player by means of a stop knob which moves approximately 10 cm. The movement is transmitted to the wind-chest by rods and bellcranks. As far as is possible, the knobs are generally grouped together opposite the keyboards to which they belong. The manual selection is often supplemented by foot-operated selection (to select mixed stops, reed-stops, etc.), to make the task of the player easier. While playing, the player often needs one stop puller (and sometimes even two) with fast reflexes, which does not simplify matters. Thus, to avoid this, makers have brought out so-called "adjustable combination" systems which simply store certain stop configurations to be called up at will. At the present time, the introduction of electrical controls which enable the stop-actuating knobs to be operated electrically has made it possible for many instruments to have up to 15 adjustable combinations, i.e. to store 15 call-up configurations for 100 or so stops and various couplings of keyboards in a memory formed from a multiplicity of bistable relays. Such an arrangement for selectively positioning the organ stops has the disadvantage of being unreliable and noisy, of consuming considerable energy, and of not allowing the combinations to be conveniently recorded in another memory device.